Each chip producer, or “foundry”, often has its own set of rules regarding the sizes of chips that can be made in that foundry. A common limit is, for example, 20×20 mm2. It is relatively difficult to form a large format image sensor, i.e., one larger than that.
Active pixel sensors have integrated amplifiers and other logic formed on the same substrate with the image sensor chip. This obviates certain problems that are associated with charge-coupled devices. The typical active pixel sensor chip has logic along at least two edges of the chip. The other edges of the chip are typically formed with “guard rings” around the edge of the image sensor.